Image-converting devices



Dec. 6, 1955 G. s. P. FREEMAN IMAGE-CONVERTING DEVICES Filed July 25, 1950 GEORGE STANLEY PERCIVAL. FREEMAN INVENTOR.

. tion.

2,726,352 IMAGE-CONVERTING DEVICES JGenrge Stanley Percival Freeman, Chiswick, London,

England, ,assigner-to -Cinema-Lelevision Limited, Lon- Idan, England, 1.a British company Applicatonjnly "25,Y 1950, Serial Nm `1175,751 claims priority, application Great Britannia/,25, 1949 6`1Claim s. {(Cl.;315'11) Thisinventioirrelatesto image-converting devices such `astelevisionpickup tubes orfthelike, and more particularly tofsuch devices -otthe-orthicon typecomprising a ftarget-electrode `carrying-'a -photosensitive mosaic `on one surface and meansfor'scanning the opposite surface with a low-velocity yelectron beam.

EWhile television pickup tubes -of the -orthicon type afford numerous Vadvantages overother'known-types of :image-converting.devices, 'faulty operation is sometimes 'encounteredfWhen-tal brilliant illuminationlis suddenly imfpresse'd -upon `the photosensitive mosaic.` `When the .rnosaic suddenly receives I brilliant illumination, as from iarphotographersfflashtlamp.or=the like,fthe potential of fthe f target electrode is tsuddenlyV driven strongly positive, :and: the. tube.Y reaches;an.unstable state ,of.` operation before the operator .is .able tto `stop down the cameraobjective lens. This effect manifestsitselftas a fogging `of `the televised image and necessitates. shuttingdownV the camera -for Va :short .time to restore the mosaic 4to `its vnormal equilibrium :potential near thatofithe `scanning `beam `cathode in` order to permit subsequent-image conversion.

4VIt is a primary object of. the present. invention-to epro- Vvide an improved image-,converting ,device of lthe orthicon gtypegin whichsuch faulty `operation Vis precluded, either in whloe or in part.

An image-converting devicew constructed in l.accordance with the present nventioncomprises a targetlelectrode including a thin layer of semi-conducting material having a photosensitive mosaic Supportedon one surface thereof. A capacitivey elect-rode of conductive material is disposed in .substantially parallel space` relation withand inl close `proximity tothe ,photosensitivemosaig and means .are Vprovided forscamiing the surface ofthe semiconductive layeropposite the mosaic with a beam of low-velocity electrons. Preferably, the capacitive electrodeis maintained atl a potential slightly positive with respect to the equilibrium potential of the target electrode to causel the capacitive electrode vto the mosaic, thereby to preventthe potential of the target electrode from rising substantiallyabove a predetermined value equal to the sum of the capacitive-electrode potential and the emission velocity of the photoelectrons originating at the mosaic. 1

Thefeatures ofthe present invention Vwhich are believed to be novel are -set forthwith particularty in the appended claims. .The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with `the accompanying drawing, in which the single ligure is a side View, partly schematic andV partly in section, of an illustrative embodiment of the invenln the drawing, certain elements of the tube are shown very much outof proportion with respect to other elements for the purpose of clearness -of illustration.

As shown in the drawing, an image-converting device constructed in accordance with the invention-comprises an evacuated-envelopetll) within which is supportedL a collect photoelectrons emitted by targetv electrode includinga Athin layerl or diaphragm "11 Y Y ,extent in that short interval.

pickup tubes, Yan Velectron envelope l0 and serves to` provide ,scanningcoils A15 -or equivalent nf ,the Scanning beam vSystem may ,-gnveiently assume 25` of envelope l0. `which may also assume theltormof-Va conductive coatingV on rthe inner wall Vofenvelopel,

YAYning electrons are returned toward lamp Y or `the 2,726,352 @Patented Dec. 6, 1955 ICC of .semi-conducting material, such `as glass or aluminum oxide which `has been suitably vtreated to renderit slightly conductive. The semi-conductive :layer 4is made Sullicientlythnfoftbe orderof 0.0002-inch, for example) to permit :leakage Vof electrical charges transversely from one ,surface to the other lin less than one Eframe kperiod (lfm-second under present televisionstandards) but does not permit lateral charge conduction -to any appreciable A mosaic vl1,2 of photosensitive material vis disposed `on one Ysurface of semiconductive `layer 1,1 to complete the target electrode structure. The composite target electrode may be ofthe type conventionally used ,in `ordinary orthicon pickup tubes and may be supported Within envelope 1 0 byany suitable means (not shown), asforiexamp l e, by means of l a Aresilient `mounting ring Aperipherally engaging :the inner wallof the envelope. Asi-,iu `c onventio1-1al orthicon gun 13 ,is supported within anelectron beam which is collimated by meansnof a long solenoid v14 surrounding ,envelope 11i?, and which is causedtoscan the surfaceof the target electrode opposite mosaic '1 2 by electromagnetic means. The` nal anode the-form of aconductivecoating 16 on the inner wall li/loreovena decelerating electrode 1.7,

v is provided between accelerating electrode VV16 and ,semi-conducting Ylayer ,11

.toy reduce the ,beam -velocitytoa yaluenearvgeroasjhe .electrons approachk the target electrode.

T hat 4 portion of the illustrated embodiment thus Y far described-corresponds essentially to lar conventional ot'h'ilcenupickup tube of` a type well known in the iart `Wlien lightfrom an Voptical image 18 isdirected onto photosensitive mosaic 12, as. by means lof a lens system'repi-esented. schematically by .lated charge Vimage-V is produced on -thetarget electrode byyirtue 1of the electron-deficiency resulting from photo- `emission from mosaic 12. inproportion ,to the intensity :asngle lens .19, a space-modu- ,ofillumination at each picture point. When the opposite `surface of V,the `target electrode ,i'sscannedwithfalowvelocity electron beam fromelectron ,gun 13, the ,scanthe electron gun in proportion to the local charge l,atuhe point where the scanning beam approaches the targetelectrode. '.vIn order `toplrtain an Voutput signalrofuseful amplitude, it is customary to include an electron multiplier arrangement w (not shown) in the electron gun 13. 501 exampl`e,rthe formillustrated and .pending application of George `Stanley PercivalV Freeman, ,Serial No. 122,925, filed October 22, 1949, 'for Such electron multipliers are well known-to the artand mayassurnegfor described inr the cov,I'argfet Electrodes for Use in Television Pickup Tubes or'rthe l i,ke,and. assigned to the present assignee.

While a conventional Vorthicon of the type described ,provides quite satisfactory Vimage conversion1 under most operatingconditions, it has beenfoundthat `sudden bril- Aliant-illumination of the Vphotosensitivel mosaic, which rrrlaynforexample be causedbyfa photographersitlash like, causes the` equilibrium potentialof the target electrode to rise above Athat of the cathode ofthe scanningzelectron gun, thereby causing fogging of nthe `televised image.

ylnpaccordance with the present invention, a capacitive electrodef20 of conductive material is disposed inV substantially parallel space relation with, and in close proximity to, photosensitive mQsaicMlZand iswlmaintaiiiedfat a potential slightly positive with respect to lth'e 'desired target electrode equilibrium potential, in order substantially. t0` prevent ,instability .and @gaius in reS'PQPSe, .t0 ,Sudden brilliant; illumination ,ofthe assai@ f'lrsfelflly,

yof electron gun 13 by the action of the capacitive electrode light-transparent conductive film or a very fine conductive mesh structure supported on the inner surface of the endy wall of envelope 10, so that light from the optical image 18 may be directed on photosensitive mosaic 12 through the capacitive electrode 20. Electrode 20 may be maintained at the desired potential by connecting it to a variable tap 21 on a potentiometer 22 connected in parallel with a suitable bias voltage source, here represented as a battery 23, one terminal of which is grounded. An output load resistor 24 may be included in the circuit between capacitive electrode 20 and variable tap 21. The cathode of electron gun 13 may conveniently be grounded, as by means of lead 25, to provide a reference Vpotential with respect to which the potential of capacitive electrode 20 may be adjusted.

In operation, in the absence of external light directed onto the photosensitive mosaicV 12, the target electrode is maintained at a potential very near that of the cathode low-velocity scanning electrons. When light is focussed onto mosaic 12 from image 18, photoelectrons are emitted from the mosaic in proportion to the local light intensity and are collected by capacitive electrode 20 which is maintained slightly positive with respect to the gun cathode. By disposing the target electrode with photosensitive mosaic'12 in substantially parallel relation with, and in close proximity to, capacitive electrode 20, saturation of the photoemission is obtained with only one or two volts positive potential applied to electrode 20. When any element of photosensitive mosaic 12 rises to a potential equal to the sum of the potential of electrode 2i) and the lemission velocity (about 11/2 volts), space charge saturation occurs'in the space between mosaic 12 and electrode 20, and no further potential rise of the target electrode may be effected. Thus, the equilibrium potential of the target electrode can never rise above a poten- ,tial corresponding to the sum of the potential applied to electrode 20 and the'v emission velocity of the photoelectrons, even under the brightest illumination. By maintaining the potential of electrode 20 at a low positive value with respect to the desired target electrode equilibrium potential, operation such caused by impact of the scanning beam remains always below unity ratio with respect to the primary electrons is insured. Thus, the potential of semi-conducting layer 11 is stabilized, and instability and fogging of the televised image are avoided.

Output signals may be derived in the conventional manner by means of an electron multiplier included in electron gun 13 for collecting and amplifying the return electron beam, or, if desired, by means of a load im pedance 24 connected to capacitive electrode 20. In the' latter instance, the output signal is developed by the load impedance connected to Velectrode 20 by virtue of the capacitive action of that electrode with respect to photosensitive mosaic 12.

Thus, the invention provides a new and improved image-converting device of the orthicontype in which instability is substantially avoided by preventing the equilibrium potential of the target electrode from rising more than a predeterminedl amount above the cathode of the-scanning electron gun.

potential of the In this manner,

lfogging of the televised image is avoided, even under conditions ofthe most brilliant illumination.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and v it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1An image-converting device comprising: an imperforate target velectrode including a thin layer of semii conducting materlal having aphotosensitive mosaic supthat secondary emissionl 20 may assume the form of a thin CII portedon one surface thereof; a capacitive electrode of conductive material disposed in substantially parallel space relation with and in close proximity to said photosensitive mosaic; means for scanning the surface of said layer opposite said one surface with a beam of lowvelocity electrons; and means for maintaining said capacitive electrode at a potential slightly -positive with respect to the equilibrium potential of said target electrode to cause said capacitive electrode to collect photoelectrons emitted by said mosaic, whereby the potential of said target electrode is prevented from rising substantially above a predetermined value equal to the sum of said first-mentioned potential and the emission velocity of said photoelectrons.

2. An image-converting device comprising: an imperforate target Velectrode lincluding a thin layer of semiconducting material having a photosensitive mosaic supported on one surface thereof; a light-permeable capacitive electrode of conductive material disposed in substantially parallel space relationl with and in close proximity to said photosensi'ive mosaic; means for projecting an optical image through said capacitive electrode onto said photosensitive mosaic to produce a space-modulated charge image on said target electrode; means for scanning the surface of said layer opposite said one surface with a beam of low-velocity electrons; and means for maintaining said capacitive electrode at a potential slightly positive with respect to the equilibrium potential of said target electrode to cause said capacitive electrode to collect photoelectrons emitted by said mosaic, whereby the potential of said target electrode is prevented from rising substantially above a predetermined value equal to the hub of said first-mentioned potential and the enlission velocity of said photoelectrons.

3. An image-converting device comprising: an imperforate target electrode including a thin layer of semi- Conducting material having a photosensitive mosaic sup- .ported on one surface thereof; a light-permeable capaciimity to said photosensitive mosaic; means for projecting an optical image through said capacitive electrode onto said photosensitive mosaic to produce a space-modulated charge image on said target electrode; means for scanning the surface of said layer opposite said one surface with a beam of low-velocity electrons; means for maintaining said capacitive electrode at a potential slightly positive with respect to the equilibrium potential of said target electrode to cause said capacitive electrode to collect photoelectrons emitted by said mosaic, whereby the potential of said target electrode is prevented from rising substantially above a predetermined value equal to the sum of said first-mentioned potential and the emission velocity of said photoelectrons; and a load impedance coupled to said capacitive electrode for deriving electrical output signals representing said space-modulated charge image in response to scansion of said target electrode by said beam.

4. An image-converting device comprising: an imperforate target electrode including a thin layer of semiconducting material having a photosensitive mosaic supported on one surface thereof; a capacitive electrode of conductive material disposedr in substantially parallel space relation with and in close proximity to said photosensitive mosaic; and means for scanning the surface of said layer opposite said one surface with a-bearn of lowvelocity electrons.

5. An image-converting device comprising: an imperforate target electrode including a thin layer of semiconducting material having a photosensitive mosaic supported on one surface thereof; a light-transparent conductive'iilm disposed in substantially parallel space relation with and in close proximity to said photosensitive mosaic; and means for scanning the surface of said layer 5 6 opposite said one surface with a beam of low-velocityl References Cited inthe le of this patent elegtrzrhs. image con erting devic co prsing a im UNITED STATES PATENTS v e m 1 n sensitive mosaic; and means for scanning the surface of said layer opposite said one surface with a beam of low- 10 velocity electrons. 

